Substrate for composite membrane

ABSTRACT

A substrate for a composite membrane includes a microporous polyolefin membrane for carrying a hydrophilic resin compound within the pores of the microporous membrane wherein: the average pore diameter is 1 nm to 50 nm; the porosity is 50% to 78%; the membrane thickness is 1 μm to 12 μm; and, when a mixed solution of ethanol and water (volume ratio 1/2) is dripped onto a surface of the microporous polyolefin membrane which has not undergone hydrophilization treatment, the contact angle θ1 between the droplet and the surface is 0 to 90 degrees 1 second after the dripping, and the contact angle θ2 between the droplet and the surface is 0 to 70 degrees 10 minutes after the dripping, and the rate of change of the contact angle ((θ1−θ2)/θ1×100) is 10 to 50%.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of application Ser. No. 16/319,661filed Jan. 22, 2019, which is a National Stage of InternationalApplication No. PCT/JP2017/020367, filed on May 31, 2017, which claimspriority from Japanese Patent Application No. 2016-145738, filed on Jul.25, 2016. The disclosure of application Ser. No. 16/319,661 isincorporated herein by reference in its entirety.

FIELD

The present invention relates to a substrate for a composite membrane.

BACKGROUND

Substrates for composite membranes are support materials (support body)to which a function that is not provided in the substrate itself isadded by means of the application, bonding, lamination, impregnation, orloading of a specific material and include, for example, non-wovenfabrics made from polyester or polyolefin, non-porous resin films astypified by polyester films, or porous films as typified by porouspolyolefin resin films.

Composite membranes, in which a waterproof functional layer is formed ona porous substrate made of paper with natural pulp as the mainingredient, non-woven fabrics made from a polymeric material, or aporous polymer film, have been devised for moisture permeable waterproof membranes that are permeable to water vapor (moisture) whileblocking the passage of liquid water and are used as materials toprevent clamminess in, for example, clothing or special protectiveclothing, and sanitary goods, (e.g., disposable diapers). As a materialfor sanitary goods, for example, Patent Literature 1 discloses amaterial made by laminating a polyolefin-based synthetic resin membraneonto a material such as a non-woven fabric made from polypropylene whichhas favorable gas permeability and moisture permeability. Furthermore,for example, in Patent Literature 2, a moisture permeable waterproofmembrane with gas barrier properties, moisture permeability, andthinness is achieved by providing a hydrophilic resin coating layer on aporous polyolefin membrane.

Moreover, for example, for a polymer electrolyte of a fuel cell, asproposed in Patent Literature 3, by including a polymer electrolyte invoids (pores) of a porous polymer membrane, an improvement in mechanicalstrength is achieved that could not be attained by the electrolyteitself. Furthermore, for example, Patent Literature 4 proposes a poroussubstrate suitable for holding a polymer electrolyte within a porouspolyethylene membrane. Patent Literature 5 discloses a thin electrolytemembrane with excellent dynamic strength obtained by the uptake andincorporation of an ion exchange resin into a thin, porous, very highmolecular weight polyolefin membrane mesh structure. Patent Literature 6discloses a thin electrolyte membrane with excellent dynamic strengthobtained by the uptake and incorporation of an ion conductive body intoa solid porous polymer membrane using capillary condensation action.

Furthermore, for example, in a total heat exchange element, a supplypath and a discharge path are formed into individual paths by providinga separation plate or a separation membrane therebetween. In relation tosuch heat exchange elements, Patent Literature 7 achieves a resin totalheat exchange element provided with gas barrier properties and moisturepermeable properties by providing a separation plate by forming awater-insoluble porous membrane comprising a water-insoluble hydrophilicmoisture permeable resin having gas barrier properties on a poroussubstrate (non-woven fabric) such as a non-woven fabric.

Recently, composite membranes in which a hydrophilic resin compound,etc., is impregnated (loaded) into the fine pores of a porous polyolefinmembrane have been proposed which are used in various applications suchas the moisture permeable water proof membranes and fuel cell polymerelectrolytes mentioned above. In order to impregnate (load) the porouspolyolefin membrane with a hydrophilic resin, a method involving theimpregnation of an aqueous solution of a hydrophilic resin compounddissolved in water into the porous polyolefin membrane is typical. Forexample, various methods of impregnating an electrolyte solution into aporous membrane with pores to obtain a proton conductive membrane havebeen proposed, and Patent Literature 8 proposes impregnating a porouspolymer membrane with an electrolytic solution comprising a phosphateand an organic solvent to obtain a thin membrane electrolyte for a roomtemperature fuel cell.

However, in porous membranes made from hydrophobic resins typified bypolyolefin, if hydrophilization treatment is not implemented on theporous membrane, water repellency of the porous membrane is great in theabove method, and as aqueous solutions of a hydrophilic resin compound(e.g., the electrolyte) are repelled, aqueous solutions of thehydrophilic resin compound do not permeate the porous membrane and sothe hydrophilic resin compound cannot be loaded into the porousmembrane.

Methods for loading a porous polyolefin membrane with an aqueoussolution of a hydrophilic resin compound include: hydrophilizationpretreatment of the porous membrane; reducing the surface free energy ofthe aqueous solution of a hydrophilic resin compound; increasing thepore diameter and porosity of the porous polyolefin membrane; andloading the aqueous solution of a hydrophilic resin compound under ahigh-pressure atmosphere or a low-pressure atmosphere.

Methods of applying hydrophilization treatment to the porous membraneinclude: chemical surface treatment (applying a surfactant onto theporous membrane, etc., (e.g., Patent Literature 9)); physical surfacetreatment (e.g., plasma treatment or corona treatment); and moisturepretreatment with an alcohol and the like. However, for chemical surfacetreatment, problems including the mixing of unwanted substances such asa surfactant into the hydrophilic resin compound in the porouspolyolefin membrane may occur, in addition to an increase in productioncosts. As degradation of the porous polyolefin membrane (cleavage ofpolymer strands) due to the treatment occurs in the case of physicalsurface treatment, problems including weakening of the porous membraneitself and a decrease in dynamic strength arise. In particular, thethinner the porous polyolefin membrane is, the more serious the decreasein dynamic strength. Furthermore, the smaller the pore diameter of theporous polyolefin membrane, the harder it is for the hydrophilictreatment effect to fully reach the interior of the porous membrane. Asthe interior of the porous polyolefin membrane is soaked in an alcoholin wetting pretreatment with an alcohol, problems such as unnecessaryalcohol being mixed into the hydrophilic resin compound and hindering ofthe permeation of the aqueous solution of the hydrophilic resin compoundarise. In order to solve these problems, it is necessary to provide asubstrate that can be impregnated with an aqueous solution of ahydrophilic resin compound without applying the aforementionedhydrophilic treatment.

Methods for reducing the surface free energy of an aqueous solution of ahydrophilic resin compound include adding alcohol to the aqueoussolution of a hydrophilic resin compound, and by lowering the surfacefree energy, impregnation into the porous polyolefin membrane isfacilitated. However, if the alcohol concentration within the aqueoussolution increases, the solubility of the hydrophilic resin compounddecreases, and the concentration of the resin compound cannot besufficiently maintained, further problems include the environmental loadthat is involved during production. In order to solve such problems, asubstrate which can be impregnated with a solvent having a low alcoholconcentration is required.

Methods for increasing the pore diameter or porosity of the porouspolyolefin membrane include methods in which a foaming agent is addedduring the production of the porous polyolefin membrane to activelyincrease or enlarge the pores, or methods in which the amount blended ofa nucleating agent, which become the pores, is increased. However, ifsuch production methods are adopted, there are problems such as asignificant reduction in dynamic strength of the porous polyolefinmembrane itself leading to deterioration in productivity due to membranetearing during production. Furthermore, even if porous polyolefinmembranes could be produced, there is the same problem of membranetearing in the step of impregnation with an aqueous solution of ahydrophilic resin compound. Moreover, there is the problem of thehydrophilic resin peeling or coming off the porous polyolefin membraneafter the solvent has been removed after impregnation with the aqueoussolution. In order to solve such problems, it is necessary to increasethe surface area of the polyolefin, which is the porous membranematerial, in contact with the hydrophilic resin, and furthermore it isnecessary to provide a substrate in which the hydrophilic resin can befinely dispersed into the polyolefin porous membrane to the maximumextent to increase local homogeneity (equivalent porosity but moremicroporous).

Problems that occur in methods for loading an aqueous solution of ahydrophilic resin compound under a high-pressure environment or alow-pressure environment include complicated production processesleading to increased costs, as well as tearing of the membrane duringproduction, and loading deficiencies of the aqueous solution of ahydrophilic resin compound. In order to address these problems, it isnecessary to provide a substrate into which an aqueous solution of ahydrophilic resin compound can permeate under atmospheric pressure.

CITATION LIST Patent Literature

-   [PTL 1] Japanese Unexamined Patent Publication (Kokai) No.    H08-141013-   [PTL 2] Japanese Unexamined Patent Publication (Kokai) No.    2014-61505-   [PTL 3] Japanese Unexamined Patent Publication (Kokai) No.    2005-166557-   [PTL 4] Japanese Unexamined Patent Publication (Kokai) No.    2011-241361-   [PTL 5] Japanese Unexamined Patent Publication (Kokai) No. 64-22932-   [PTL 6] Japanese Unexamined Patent Publication (Kokai) No. H1-158051-   [PTL 7] Japanese Unexamined Patent Publication (Kokai) No.    2008-032390-   [PTL 8] Japanese Unexamined Patent Publication (Kokai) No. H08-88013-   [PTL 9] Japanese Unexamined Patent Publication (Kokai) No.    H01-186752

SUMMARY Technical Problem

The object of the present invention is to provide a substrate for acomposite membrane comprising a microporous polyolefin membrane thepores of which can be satisfactorily loaded with a hydrophilic resincompound and which has favorable permeability to an aqueous solutionwith a high water concentration and a comparatively high surface freeenergy, without hydrophilization pretreatment being carried out thereon.

Solution to Problem

In order to solve the above problems, according to the presentinvention, the following configurations are adopted.

[1] A substrate for a composite membrane comprising a microporouspolyolefin membrane for carrying a hydrophilic resin compound withinpores of the microporous membrane wherein: the average pore diameter is1 nm to 50 nm; the porosity is 50% to 78%; the membrane thickness is 1μm to 12 μm; and, when a mixed solution of ethanol and water (volumeratio 1/2) is dripped onto a surface of the microporous polyolefinmembrane which has not undergone hydrophilization treatment, a contactangle θ1 between the droplet and the surface is 0 to 90 degrees 1 secondafter the dripping, and a contact angle θ2 between the droplet and thesurface is 0 to 70 degrees 10 minutes after the dripping, and a rate ofchange of the contact angle ((θ1−θ2)/θ1×100) is 10 to 50%.

[2] The substrate for a composite membrane according to [1], wherein therate of change of the contact angle ((θ1−θ2)/θ1×100) is 17 to 41%.

[3] The substrate for a composite membrane according to [1] or [2],wherein the polyolefin is a polyethylene composition comprising a highmolecular weight polyethylene with a mass-average molecular weight of900,000 or more and a low molecular weight polyethylene with amass-average molecular weight of 200,000 to 800,000 mixed at a massratio of 20:80 to 80:20.

[4] The substrate for a composite membrane according to any one of [1]to [3], wherein the substrate for a composite membrane can beimpregnated with a liquid which is a solvent for the hydrophilic resincompound and has a surface free energy of 35 to 36.5 mJ/m².

[5] The substrate for a composite membrane according to [4], wherein thesubstrate for a composite membrane can be impregnated with a mixedsolution of ethanol and water in which the water concentration isgreater than 65.8% by volume but no greater than 70.6% by volume.

[6] The substrate for a composite membrane according to any one of [1]to [5], wherein the Gurley value as measured according to JIS P8117 is90 s/100 cc or less.

[7] The substrate for a composite membrane according to any one of [1]to [6], wherein the tensile breaking strength (MD or TD) per unit crosssectional area of the polyolefin solid content is 50 MPa or more.

Advantageous Effects of Invention

According to the present invention a substrate for a composite membranecomprising a microporous polyolefin membrane the pores of which can besatisfactorily loaded with a hydrophilic resin compound and which hasfavorable permeability to an aqueous solution with a high waterconcentration and a comparatively high surface free energy, withouthydrophilization pretreatment being carried out thereon can be provided.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph comparing the relative humidity dependency of protonconductivity of a Membrane Electrode Assembly (MEA) that uses thesubstrate for a composite membrane of the present invention, and aconventional electrolyte membrane.

FIG. 2 is a graph comparing the current density dependency of cellvoltage of the MEA that uses the substrate for a composite membrane ofthe present invention and a conventional electrolyte membrane.

DESCRIPTION OF EMBODIMENTS

The following embodiments of the present invention will be described inorder. These explanations and embodiments are for exemplifying theinvention and do not limit the scope of the invention. Note that, in theentire description, numerical ranges expressed by “to” include the uppervalue and lower value. Further, with regard to the microporouspolyolefin membrane, “longitudinal direction” refers to the direction ofthe length of the microporous polyolefin membrane that is produced in anelongated shape, and “transverse direction” refers to a direction thatis perpendicular to the longitudinal direction of the microporouspolyolefin membrane. Below, “transverse direction” may be referred to as“TD” and the “longitudinal direction” may be referred to as “MD”.

[Substrate for Composite Membrane]

The substrate for a composite membrane of the present inventioncomprises a microporous polyolefin membrane and is for carrying ahydrophilic resin compound within the pores of the microporous membranewherein the average pore diameter is 1 nm to 50 nm, the porosity is 50%to 78%, the membrane thickness is 1 μm to 12 μm, and when a mixedsolution of ethanol and water (volume ratio 1/2) is dripped onto asurface of the microporous polyolefin membrane which has not undergonehydrophilization treatment, a contact angle θ1 between the droplet andthe surface is 0 to 90 degrees 1 second after the dripping, and acontact angle θ2 between the droplet and the surface is 0 to 70 degrees10 minutes after the dripping, and a rate of change of the contact angle((θ1−θ2)/θ1×100) is 10 to 50%. The details of each configuration will beexplained below.

(Average Pore Diameter)

The average pore diameter of the microporous polyolefin membrane whichis a substrate for a composite membrane of the present invention is 1 nmto 50 nm. When the average pore diameter of the microporous polyolefinmembrane is 50 nm or less, even if the porosity of the microporousmembrane is high, the microporous polyolefin membrane becomes favorablein terms of dynamic strength, and handleability is improved.Furthermore, for a given porosity, the smaller the average pore diameterthe greater the frequency of pores present in the microporous membrane,which allows even loading of the hydrophilic resin compound over theentire porous polyolefin membrane. Furthermore, as the frequency ofpores present over the surface of the microporous membrane increases,the more favorable the permeability to solvents used for the hydrophilicresin compound which have a higher surface free energy. From such aviewpoint, it is preferable for the microporous polyolefin membrane tohave an average pore diameter of 45 nm or less, and more preferably 40nm or less. When the average pore diameter is 1 nm or more, thepermeation speed of solvents with a high surface free energy isimproved. From such a viewpoint, it is preferable for the microporouspolyolefin membrane to have an average pore diameter of 5 nm or more,and more preferably 10 nm or more.

The average pore diameter of the microporous polyolefin membrane can bemeasured by the methods described in the examples below.

(Porosity)

The microporous polyolefin membrane which is a substrate for a compositemembrane of the present invention has a porosity of 50 to 78%. Aporosity of 50% or more is desirable for the microporous polyolefinmembrane because the loading rate of the hydrophilic resin compoundbecomes high and the properties of the resin compound itself can besufficiently expressed, and also because permeation of a solution, inwhich the hydrophilic resin compound is dissolved, into the microporousmembrane is facilitated and the rate of permeation is accelerated. Fromsuch a viewpoint, a microporous polyolefin membrane porosity of 55% ormore is preferable, and more preferable is 60% or more. On the otherhand, when the porosity is 78% or less, the dynamic strength of themicroporous polyolefin membrane becomes favorable which is desirable interms of improved handleability. From such a viewpoint, a microporouspolyolefin membrane porosity of 75% or less is preferable, and morepreferable is 66% or less.

The porosity (ε) of the microporous polyolefin membrane can be measuredby the measuring methods described below in the Examples and iscalculated with the following formula.

ε(%)={1−Ws/(ds·t)}×100

Ws: weight of microporous polyolefin membrane (g/m²)ds: true density of polyolefin (g/cm³)t: thickness of microporous polyolefin membrane (μm)

(Membrane Thickness)

The microporous polyolefin membrane which is a substrate for a compositemembrane of the present invention has a thickness of 1 μm to 12 μm. Whenthe thickness of the microporous polyolefin membrane is 1 μm or more,sufficient dynamic strength can be easily attained, making it desirablein terms of handleability during the processing of the microporouspolyolefin membrane or in terms of allowing stable transportationthereof when impregnating the solution of the dissolved hydrophilicresin compound. From such a viewpoint, it is preferable for themicroporous polyolefin membrane to be 3 μm or more, and more preferably4 μm or more. On the other hand, if the thickness is 12 μm or less, thetime required for impregnation of the solution of dissolved hydrophilicresin compound into the microporous membrane becomes shorter, and thehydrophilic resin compound can be loaded evenly without any patches overthe entire microporous membrane. Moreover, when the substrateimpregnated with the hydrophilic resin compound is modularized, theefficiency per volume is desirably improved. From such a viewpoint, amicroporous polyolefin membrane thickness of 10 μm or less ispreferable, and more preferable is 9 μm or less.

In general, microporous polyolefin membranes are opaque and white due todispersion of light by the presence of pores. However, as the pores aresubstantially loaded with a hydrophilic resin compound solution, lightdispersion is reduced, and with the combined effect of the thin membranethickness, the obtained composite membrane sometimes becomessubstantially transparent over the entirety thereof

(Contact Angle)

For the microporous polyolefin membrane which is a substrate for acomposite membrane of the present invention, when a mixed solution ofethanol and water (volume ratio 1/2) is dripped onto a surface of themicroporous polyolefin membrane which has not undergone hydrophilizationtreatment, the contact angle between the droplet and the surface is 0 to90 degrees 1 second after the dripping. When the contact angle after 1second is 90 degrees or less, there is a synergistic effect with theporous structure having the above porosity and average pore diametersuch that permeation of the solution of dissolved hydrophilic resincompound into the microporous membrane is facilitated. From such aviewpoint, a contact angle after 1 second of 88 degrees or less ispreferable, and more preferable is 85 degrees or less.

Furthermore, for the microporous polyolefin membrane which is asubstrate for a composite membrane of the present invention, when amixed solution of ethanol and water (volume ratio 1/2) is dripped onto asurface of the microporous polyolefin membrane which has not undergonehydrophilization treatment, it is desirable that the contact anglebetween the droplet and the surface is 0 to 70 degrees 10 minutes afterthe dripping. When the contact angle of the microporous polyolefinmembrane is 70 degrees or less, the solution of dissolved hydrophilicresin compound can more easily permeate the microporous membrane, makingit desirable in terms of sufficiently loading the microporous membranewith the compound. From such a viewpoint, a contact angle after 10minutes of 65 degrees or less is preferable, and more preferable is 60degrees or less.

The contact angle can be measured using the measurement methodsdescribed below in the examples.

Note that, when the ethanol-water solution is dripped on the microporouspolyolefin membrane of the present invention, the droplet does notspread outward in the radial direction, but rather the droplet exhibitsthe behavior of maintaining the diameter thereof or shrinking inward inthe radial direction when permeating the membrane.

(Rate of Change of Contact Angle)

The microporous polyolefin membrane according to the present inventioncan also be defined from the viewpoint of the change over time of thecontact angle. Namely, when a mixed solution of ethanol and water(volume ratio 1/2) is dripped onto a surface of the microporouspolyolefin membrane which has not undergone hydrophilization treatment,it is desirable that a contact angle θ1 between the droplet and thesurface be 0 to 90 degrees 1 second after the dripping, and a contactangle θ2 between the droplet and the surface be 0 to 70 degrees 10minutes after the dripping, and a rate of change of the contact angle((θ1−θ2)/θ1×100) be 10 to 50%. When the rate of change of the contactangle is 10% or more, it is considered that the impregnation rate of theelectrolyte solution into the microporous polyolefin membrane issufficient from the viewpoint of practical production efficiency. Fromsuch a viewpoint, it is preferable for the rate of change of contactangle to be 15% or more, more preferably 17% or more. On the other hand,from the viewpoint of maintaining sufficient dynamic strength of themicroporous polyolefin membrane, it is preferable for the rate of changeof the contact angle to be 45% or less, more preferably 41% or less.

The present invention has an extremely small average pore diameter of 1nm to 50 nm as well as a comparatively high porosity of 50% to 78%, andhas achieved an extremely thin membrane thickness of 1 μm to 12 μm. Sucha microporous polyolefin membrane could not conventionally be obtained.Furthermore, by combining the aforementioned surface properties of thecontact angle between the droplet and the surface being 0 to 90 degrees1 second after the dripping, the contact angle between the droplet andthe surface being 0 to 70 degrees 10 minutes after the dripping, and therate of change of the contact angle being 10 to 50%, the permeability toan aqueous solution with a high water concentration and surface freeenergy becomes a favorable level that could not be conventionallyachieved, and thereby a high concentration of hydrophilic resin compoundcan be loaded into the pores.

Note that in the present invention, it is necessary to adjust theaverage pore diameter, porosity, and contact angle of the aforementionedmicroporous polyolefin membrane substrate to a suitable range. The meansby which these physical properties are controlled is in no way limited.However, production conditions can be adjusted for: the averagemolecular weight of the polyethylene resin; the mixing ratio when aplurality of polyethylene resins are mixed and used; the polyethyleneresin concentration in the raw material; the mixing ratio of solventsmixed into the raw material when a plurality thereof are mixed and used;the stretch ratio and the heat treatment (heat setting) temperatureafter stretching; and the soaking time in an extraction solvent, etc. Inparticular, as will be indicated in the production method below, it ispreferable that: the mass ratio of a high molecular weight polyethylenein the entire polyethylene composition be 20 to 80% by mass; that thepolyethylene resin in the raw material comprise 5% by mass or more of ahigh molecular weight polyethylene with a mass-average molecular weightof 900,000 or more; that a mixture of a volatile solvent and anonvolatile solvent be used as a solvent of the polyolefin solution(content of nonvolatile solvent in the whole solvent being 80 to 98% bymass); that the stretch ratio of the entirety be a ratio of 45 to 100;and that the heat setting temperature be 120 to 135° C.

(Surface Free Energy of Liquid that can be Used for Impregnation)

The microporous polyolefin membrane which is a substrate for a compositemembrane according to the present invention can be impregnated withliquid that is a hydrophilic resin compound solvent with a surface freeenergy of 35 to 36.5 mJ/m². The surface free energy of a liquid as usedherein is a value measured at 20° C.

When the surface free energy of this liquid is 36.5 mJ/m² or less, theaffinity between the solvent that dissolves the hydrophilic resincompound and the polyolefin resin that forms the microporous membranebecomes higher which is desirable from the point that it is easier forthe solution in which the hydrophilic resin compound has dissolved topermeate the microporous membrane. Moreover, when the surface freeenergy is 35 mJ/m² or more, the concentration of the hydrophilic resincompound in the solvent can be increased, which is desirable from thepoint of increasing the loading efficiency of the hydrophilic resin intothe microporous membrane. Note that, the surface free energy can be setto the aforementioned range by, for example, using a mixed solution ofan alcohol (e.g., methanol, ethanol, isopropanol, or t-butyl alcohol)and water, or a mixed solvent obtained by mixing an organic solvent suchas ethylene glycol, tetrahydrofuran, acetone, methyl ethyl ketone,dimethylformamide, triethylamine, etc., with water in a required range.

Note that a method for adjusting the surface free energy of theaforementioned liquids can be achieved by, for example, mixing a volumeratio of ethanol of 29.5% by volume or more and less than 34.2% byvolume into water (namely, the water concentration is more than 65.8% byvolume but not more than 70.5% by volume).

(Hydrophilic Resin Composition)

A resin compound that is soluble in a liquid with a surface free energyof 35 to 36.5 mJ/m² can be used as the hydrophilic resin compound thatis suitably carried by the microporous polyolefin membrane which is asubstrate for a composite membrane according to the present invention.For example, as the hydrophilic resin compound a polymer withhydrophilic groups such as a sulfonic acid group, carboxyl group, orhydroxyl group is preferable. More specific examples include,perfluorosulfonic acid type resins, polyvinyl pyrrolidone, polyvinylalcohol, polyethylene glycol, polyacrylic acid, polymethacrylic acid,polystyrenesulfonic acid, celluloses and the like. A modified compoundof the resin compound or a dispersion of fine aggregates of the resincompound, for example, may be used provided the compound does not becomeonly sparingly soluble when the aforementioned liquid is used to preparean aqueous solution of the hydrophilic resin compound. These hydrophilicresin compounds are preferred because they have high solubility in asolvent containing water so that impregnation of the resin compound intothe microporous polyolefin membrane can be efficiently performed and theresin compound does not peel off or fall off from the microporouspolyolefin membrane after impregnation and can be carried well.

(Gurley Value)

The microporous polyolefin membrane which is a substrate for a compositemembrane according to the present invention has a Gurley value, asmeasured according to JIS P8117, of preferably 90 s/100 cc or less, morepreferably 85 s/100 cc or less, and even more preferably of 75 s/100 ccor less. When this Gurley value is 90 s/100 cc or less, the solution inwhich the hydrophilic resin compound is dissolved permeates themicroporous membrane more easily which is desirable from the point thatthe impregnation speed becoming faster.

(Tensile Breaking Strength)

The microporous polyolefin membrane which is a substrate for a compositemembrane of the present invention has a tensile breaking strength in atleast one of the longitudinal direction (MD) and the transversedirection (TD) of preferably 50 MPa or more, and more preferably 60 MPaor more. When the strength of the microporous polyolefin membrane is 50MPa or more, the dynamic strength for the composite membrane becomesfavorable and the point that the handleability improves in the step ofimpregnating the microporous polyolefin membrane with an aqueoussolution of hydrophilic resin compound is desirable.

(Polyolefin)

The microporous polyolefin membrane which is a substrate for a compositemembrane according to the present invention is a microporous membranecomposed of polyolefin. The microporous membrane has many microporestherein and has a structure wherein these micropores are interconnected,meaning that gas or liquid can pass from one surface to the othersurface. It is preferable for the microporous polyolefin membrane tocomprise polyolefin at 90% by mass or more, more preferably 95% by massor more and the remainder may include additives such as organic orinorganic fillers or surfactants in amounts limited so as not toinfluence the effects of the invention.

The polyolefin may be, for example, a homopolymer or a copolymer ofpolyethylene, polypropylene, polybutylene, or polymethylpentene, or maybe a mixture of one or more thereof, and among these, polyethylene ispreferable. Low molecular weight polyethylene or a mixture of lowmolecular weight polyethylene and high molecular weight polyethylene issuitable as the polyethylene. Furthermore, polyethylene may be used incombination with another component. Examples of components other thanpolyethylene include polypropylene, polybutylene, polymethylpentene, anda copolymer of polypropylene and polyethylene. The polyolefin may be acombination of a plurality of polyolefins having poor compatibility witheach other, having different degrees of polymerization and branchingproperties, in other words, a plurality of polyolefins having differentcrystallinity, stretching properties and molecular orientation.

The polyolefin used in the present invention is preferably apolyethylene composition comprising 5% by mass or more of a highmolecular weight polyethylene with a mass-average molecular weight of900,000 or more, more preferably a composition comprising 7% by mass ormore of the high molecular weight polyethylene, and particularly acomposition comprising 15 to 90% by mass of the high molecular weightpolyethylene. Moreover, blending a suitable amount of two or more typesof polyethylene has the effect of forming a network structure thataccompanies fibrillation upon stretching and increasing the poregeneration rate. The mass-average molecular weight after blending two ormore types of polyethylene is preferably 500,000 to 4,500,000, morepreferably 500,000 to 4,000,000. In particular, a polyethylenecomposition comprising a blend of the aforementioned high molecularweight polyethylene with a mass-average molecular weight of 900,000 ormore and a low molecular weight polyethylene with a mass-averagemolecular weight of 200,000 to 800,000 is preferable. In such cases, itis particularly preferable for the ratio of the high molecular weightpolyethylene in the polyethylene composition to be 20 to 80% by mass.The density of the low molecular weight polyethylene is preferably 0.92to 0.96 g/cm³. The upper limit value of the mass-average molecularweight of the high molecular weight polyethylene is preferably 6,000,000or less, and 5,000,000 or less is particularly preferable. The lowerlimit value of the mass-average molecular weight of the high molecularweight polyethylene is preferably 1,000,000 or more, more preferably2,000,000 or more, and 3,000,000 or more is particularly preferable.

Note that the mass-average molecular weight was determined by dissolvinga sample of the microporous polyolefin membrane in o-dichlorobenzene byheating and measuring the sample by GPC (Alliance GPC 2000, GMH 6-HT andGMH 6-HTL columns, manufactured by Waters) at a column temperature of135° C. and a flow rate of 1.0 mL/min. Molecular weight monodispersepolystyrene (manufactured by Tosoh Corporation) may be used forcalibrating the molecular weight.

(Method for Producing Microporous Polyolefin Membrane)

The microporous polyolefin membrane which is a substrate for a compositemembrane of the present invention can be favorably produced by themethod indicated below. That is, by sequentially implementing thefollowing steps the membrane can be favorably produced.

(I) A step of preparing a solution containing a polyolefin compositionand a solvent, wherein the solution contains at least a volatile solventhaving a boiling point of less than 210° C. at atmospheric pressure.(II) A step of melt-kneading the solution, extruding the melt-kneadedproduct from a die, cooling and solidifying to obtain a gel-like moldedproduct.(III) A step of stretching the gel-like molded product in at least onedirection.(IV) A step of extracting and washing the solvent from the inside of thestretched intermediate molded product.

In step (I) a solution containing the polyolefin composition and asolvent is prepared, and a solution is prepared that contains at least avolatile solvent with a boiling point of less than 210° C. atatmospheric pressure. The solution is preferably a thermo-reversiblesol-gel solution, that is, the polyolefin is solated by heating anddissolving in the solvent thereby preparing a thermo-reversible sol-gelsolution. The volatile solvent with a boiling point of less than 210° C.at atmospheric pressure in step (I) is not particularly limited providedsufficient swelling or solvation of the polyolefin can be causedthereby. However, liquid solvents such as tetralin, ethylene glycol,decalin, toluene, xylene, diethyl triamine, ethylenediamine, dimethylsulfoxide, hexane and the like are preferred, and these solvents may beused alone or in combination of two or more. Thereamong, decalin andxylene are preferred.

Furthermore, other than the aforementioned volatile solvent with aboiling point of less than 210° C. at atmospheric pressure, whenpreparing this solution, the inclusion of a nonvolatile solvent having aboiling point of 210° C. or more such as liquid paraffin, paraffin oil,mineral oil, castor oil or the like is preferable in terms offacilitating the adjustment of average pore diameter and porosity towithin the range of the present invention. In such cases, it ispreferable for the content of the nonvolatile solvent to be 80 to 98% bymass of the whole solvent.

In the solution of step (I), from the viewpoint of controlling theloading rate of the resin composition into the microporous polyolefinmembrane substrate, it is preferable for the concentration of thepolyolefin composition to be 10 to 35% by mass, more preferably 15 to30% by mass.

In step (II), the solution prepared in the step (I) is melt-kneaded, theobtained melt-kneaded product is extruded through a die and cooled andsolidified to obtain a gel-like molded product. Preferably, an extrudateis obtained by extruding through the die at a temperature range from themelting point of the polyolefin composition to the melting point+65° C.,then the extrudate is cooled to obtain a gel-like molded product.

It is preferable to form the molded product into a sheet shape. Coolingmay involve quenching in an aqueous solution or an organic solvent orcasting with a cooled metal roll, but in general, a method of quenchingin water or the volatile solvent used at the time of the sol-gelsolution is used. The cooling temperature is preferably 10 to 40° C.Note that it is preferable to prepare the gel-like sheet by providing awater stream on the surface layer of a water bath so that the mixedsolution released from the sheet, which gellified in the water bath, andfloating on the water surface does not adhere again to the sheet.

In step (II), one or more stages of preliminary heating may be carriedout after the gel-like molded product is cooled as required, and some ofthe volatile solvent may be removed from the inside of the sheet. Insuch cases, the preliminary heating temperature is preferably 50 to 100°C.

Step (III) is a step of stretching the gel-like molded product in atleast one direction. The stretching in step (III) is preferably biaxialstretching, and either sequential biaxial stretching in whichlongitudinal stretching and transverse stretching are separately carriedout, or simultaneous biaxial stretching in which longitudinalstretching, and transverse stretching are simultaneously carried out canbe suitably applied. Further, a method of stretching in the transversedirection after stretching a plurality of times in the longitudinaldirection, a method of stretching in the longitudinal direction andstretching a plurality of times in the transverse direction, and amethod of sequential biaxial stretching followed by further stretchingone or more times in the longitudinal direction and/or the transversedirection are also favorable.

The area stretch ratio (the product of the longitudinal stretch ratioand the transverse stretch ratio) in step (III), from the viewpoint ofcontrolling the permeability of a mixed solution of ethanol and water(volume ratio 1/2) into the microporous polyolefin membrane, ispreferably a ratio of 45 to 100, and more preferably a ratio of 50 to91. The stretching temperature is preferably 90 to 110° C.

In addition to the stretching step (III), thermal fixing treatment maybe performed as necessary. In such cases, the heat setting temperatureis preferably 120 to 135° C. from the viewpoint of controlling theloading rate of the resin compound into the microporous polyolefinmembrane substrate.

Step (IV) is a step of extracting and washing the solvent from theinside of the stretched intermediate molded product. In step (IV), inorder to extract the solvent from the inside of the stretchedintermediate molded product (stretched film), it is preferable toperform washing with a halogenated hydrocarbon such as methylenechloride or a hydrocarbon solvent such as hexane. It is preferable totake 20 to 180 seconds when washing by immersing in a tank containing asolvent in order to obtain a microporous polyolefin membrane from whichless residual solvent is eluted. Furthermore, in order to furtherimprove the cleaning effect, tanks are divided into a plurality ofstages, a washing solvent is poured in from the downstream side of themicroporous polyolefin membrane transporting process, a washing solventis flowed toward the upstream side of the transporting process, and itis preferable to make the purity of the washing solvent in thedownstream tank higher than that of the upstream layer. Depending on therequired performance of the microporous polyolefin membrane, heatsetting may be performed by annealing treatment. Note that the annealingtreatment is preferably carried out at 60 to 130° C., and morepreferably 70 to 125° C. from the viewpoint of transportability and thelike during the process.

The microporous polyolefin membrane of the present invention is producedvia the aforementioned steps and is characterized in that impregnationof an aqueous solution with a high surface free energy can be favorablyachieved without implementing hydrophilization treatment involvingchemical treatment (for example, coating of a surfactant, graftpolymerization using a hydrophilic functional group, wetting treatmentwith a liquid with a low surface free energy, etc.), or physicaltreatment (for example, plasma treatment, corona treatment, etc.).

By not implementing the aforementioned chemical treatment, mixing ofcontaminants can be avoided leading to reduced production costs.Furthermore, by not implementing the physical treatment, degradation ofthe resin and a reduction in dynamic strength can be prevented.

(Application)

The substrate for a composite membrane comprising a microporouspolyolefin membrane of the present invention can constitute thecomposite membrane by carrying a hydrophilic resin compound within themicropores thereof. The composite membrane can be suitably used forclothing, special protective clothing, sanitary goods (e.g., disposablediapers), moisture-permeable waterproof membranes for total heatexchange elements and the like, and electrolyte membranes for solidpolymer fuel cells, water electrolysis, and soda decomposition, etc.

EXAMPLES

The examples, comparative examples and various measuring methods of thepresent invention are described below. However, the present invention isin no way limited by these examples.

(Measuring Methods) (Polyolefin Mass-Average Molecular Weight)

The mass-average molecular weight was determined by dissolving a sampleof the microporous polyolefin membrane in o-dichlorobenzene by heatingand measuring the sample by GPC (Alliance GPC 2000, GMH 6-HT and GMH6-HTL columns, manufactured by Waters) at a column temperature of 135°C. and a flow rate of 1.0 mL/min. Molecular weight monodispersepolystyrene (manufactured by Tosoh Corporation) was used for calibratingthe molecular weight.

(Membrane Thickness)

The membrane thickness of the microporous polyolefin membrane wasdetermined by measuring 20 points with a contact type membrane thicknessmeter (Lightmatic VL-50A, manufactured by Mitutoyo Corporation) andaveraging the results. A columnar member having a diameter of 0.5 cm onthe bottom surface was used as the contact terminal. During themeasurement, adjustments were made such that a load of 0.01 N wasapplied.

(Average Pore Diameter)

The average pore diameter of the microporous polyolefin membrane wasmeasured by using a perm-porometer (model: CFP-1500 AEX) manufactured byPorous Materials Co., Ltd. and GALWICK (perfluoropolyether with surfacetension of 15.9 dyne/cm manufactured by Porous Materials Co., Ltd.) asan impregnating solution. The mean flow pore diameter (nm) wascalculated based on the half dry method specified in ASTM E 1294-89. Themeasurement temperature was 25° C. and the measurement pressure was 200kPa to 3500 kPa.

(Porosity)

The porosity (ε) of the microporous polyolefin membrane was calculatedusing the following formula.

ε(%)={1−Ws/(ds·t)}×100

Ws: weight of porous polyolefin membrane (g/m²)ds: True density of polyolefin (g/cm³)t: thickness of microporous polyolefin membrane (μm)

Note that, the weight of the microporous polyolefin membrane wasdetermined by cutting out samples of 10 cm×10 cm, measuring the massthereof, and dividing by the area.

(Contact Angle)

The static contact angle was measured using a fully automatic contactangle meter DMo-701 FE and Interface Measurement and Analysis SystemFAMAS manufactured by Kyowa Interface Science Co., Ltd as the measuringdevice. Using a microporous polyolefin membrane which had not undergonehydrophilization treatment, a 4 μL aqueous ethanol solution (industrialethanol (purity of 95%)/pure water; mixed volume ratio 1/2) was drippedonto the sample and a contact angle θ1 1 second after the dripping and acontact angle θ2 10 minutes after the dripping were measured at normalatmospheric pressure, 24° C. and 60% relative humidity.

(Gurley Value)

The Gurley value (sec/100 cc) of a microporous polyolefin membrane withan area of 642 mm² was measured according to JIS P8117.

(Tensile Breaking Strength)

A strip-shaped test piece (15 mm in width and 50 mm in length) waspulled at a speed of 200 mm/min with a tensile tester (RTE-1210manufactured by Orientec Co., Ltd.) to determine the tensile strengthwhen the test piece breaks.

(Rate of Change of Contact Angle)

The rate of change of contact angle was calculated with the followingformula using contact angle θ1 and contact angle θ2 which wererespectively obtained when the contact angle was measured 1 second and10 minutes after a liquid was dripped on the surface and was used as anindex of permeation speed. For example, when there are two samples withthe same contact angle after 1 second, a greater rate of change ofcontact angle after 10 minutes means the permeation speed is faster.

Rate of change of contact angle=(θ1−θ2)/θ1×100(%)

(Permeability of Ethanol and Water Mixture)

Various aqueous ethanol solutions were prepared by mixing pure waterwith industrial ethanol (purity of 95%) at various volume ratios. Asample was placed on and closely contacted with a piece of paper withwhich the absorption of water could be easily seen. 10 μL of theprepared aqueous ethanol solution was dripped on the sample, andpresence or absence of liquid permeation after dripping at atmosphericpressure at 24° C. and relative humidity of 60% was observed. The liquidpermeation was judged by visually checking for wetting of the piece ofpaper 1 minute after dripping. Note that, when the color of the backside of the piece of paper was discolored, it was judged that completepermeation (o) had occurred, and when it was not discolored, it wasjudged that no permeation (x) occurred because the droplet had notpenetrated to the back side. The maximum water concentration means thehighest water concentration among the water concentrations of thepermeating liquid droplets of aqueous ethanol solution (note that theethanol concentration is converted into 100% purity when calculating thewater concentration). Also, in Table 2 below, the surface free energy ofthe aqueous ethanol solution at the maximum water concentration is alsoshown.

Example 1

A polyethylene composition comprising a mixture of 12 parts by mass of ahigh molecular weight polyethylene (PE1) with a mass-average molecularweight of 4,600,000, and 3 parts by mass of a low molecular weightpolyethylene (PE2) with a mass-average molecular weight of 560,000 wasused, and a polyethylene solution was prepared so that the concentrationof the total amount of the polyethylene resin was 15% by mass by mixingwith a solvent mixture of 72 parts by mass of liquid paraffin and 13parts by mass of decalin (decahydronaphthalene) which was prepared inadvance.

Gel-like sheets (base tape) were produced by extruding this polyethylenesolution into sheets using a die at 160° C., cooling the extrudate in awater bath at 25° C. and providing a water stream on the surface layerof the water bath so that the mixed solvent released from the sheet,which gellified in the water bath, and floating on the water surface didnot again adhere to the sheet. The base tape was dried for 10 minutes at55° C. and a further 10 minutes at 95° C. to remove the decalin fromwithin the base tape. Thereafter, the base tape was stretched by a ratioof 5.5 in the longitudinal direction at 100° C., and then stretched by aratio of 13 in the transverse direction at 110° C. after which heattreatment (heat setting) at 135° C. was immediately carried out.

Next the microporous polyethylene membrane was soaked successively intwo separate tanks containing methylene chloride baths for 30 secondseach while liquid paraffin was extracted therefrom. Note that the purityof the washing solvent in the first tank (low)<in the second tank(high), wherein the first tank was on the side where soaking was startedand the second tank was on the side where soaking was finished.Thereafter, the microporous polyethylene membrane was obtained byremoving the methylene chloride by drying at 45° C., and by carrying outannealing treatment while transporting over rollers heated to 120° C.

The obtained microporous polyethylene membrane had excellentpermeability to ethanol/water=1/2 solution and was suitable as asubstrate for a composite membrane. Note that Table 1 below indicatesthe production conditions for the microporous polyethylene membrane andTable 2 indicates the physical property values and evaluation results ofthe microporous polyethylene membrane. Furthermore, the informationregarding the other examples and comparative examples have beensimilarly collated in Tables 1 and 2.

Example 2

A microporous polyethylene membrane was obtained in the same way as inExample 1 except for the following. A polyethylene compositioncomprising a mixture of 6 parts by mass of a high molecular weightpolyethylene (PE1) with a mass-average molecular weight of 4,600,000,and 24 parts by mass of a low molecular weight polyethylene (PE2) with amass-average molecular weight of 560,000 was used, and a polyethylenesolution was prepared so that the concentration of the total amount ofthe polyethylene resin was 30% by mass by mixing with a solvent mixtureof 6 parts by mass of decalin (decahydronaphthalene) and 64 parts bymass of paraffin which was prepared in advance.

A gel-like sheet was prepared by extruding this polyethylene solutioninto sheets using a die at 160° C. then cooling the extrudate in a waterbath at 25° C.

The base tape was dried for 10 minutes at 55° C. and for a further 10minutes at 95° C. to remove decalin from the base tape. Thereafter, thebase tape was stretched by a ratio of 5.5 in the longitudinal directionat 100° C. and then stretched by a ratio of 13 in the transversedirection at 110° C. after which heat treatment (heat setting) at 125°C. was immediately carried out.

The obtained microporous polyethylene membrane has excellentpermeability to ethanol/water=1/2 solution and is suitable as asubstrate for a composite membrane.

Example 3

A microporous polyethylene membrane was obtained in the same way as inExample 1 except for the following. A polyethylene compositioncomprising a mixture of 16 parts by mass of a high molecular weightpolyethylene (PE1) with a mass-average molecular weight of 4,600,000,and 4 parts by mass of a low molecular weight polyethylene (PE2) with amass-average molecular weight of 560,000 was used, and a polyethylenesolution was prepared so that the concentration of the total amount ofthe polyethylene resin was 20% by mass by mixing with a solvent mixtureof 2 parts by mass of decalin (decahydronaphthalene) and 78 parts bymass of paraffin which was prepared in advance.

A gel-like sheet was prepared by extruding this polyethylene solutioninto sheets using a die at 160° C. then cooling the extrudate in a waterbath at 25° C.

The base tape was dried for 10 minutes at 55° C. and for a further 10minutes at 95° C. to remove decalin from the base tape. Thereafter, thebase tape was stretched by a ratio of 3.9 in the longitudinal directionat 100° C. and then stretched by a ratio of 13 in the transversedirection at 100° C. after which heat treatment (heat setting) at 135°C. was immediately carried out.

The obtained microporous polyethylene membrane has excellentpermeability to ethanol/water=1/2 solution and is suitable as asubstrate for a composite membrane.

Example 4

A microporous polyethylene membrane was obtained in the same way as inExample 1 except for the following. A polyethylene compositioncomprising a mixture of 16 parts by mass of a high molecular weightpolyethylene (PE1) with a mass-average molecular weight of 4,600,000,and 4 parts by mass of a low molecular weight polyethylene (PE2) with amass-average molecular weight of 560,000 was used, and a polyethylenesolution was prepared so that the concentration of the total amount ofthe polyethylene resin was 20% by mass by mixing with a solvent mixtureof 2 parts by mass of decalin (decahydronaphthalene) and 78 parts bymass of paraffin which was prepared in advance.

A gel-like sheet was prepared by extruding this polyethylene solutioninto sheets using a die at 160° C. then cooling the extrudate in a waterbath at 25° C.

The base tape was dried for 10 minutes at 55° C. and for a further 10minutes at 95° C. to remove decalin from the base tape. Thereafter, thebase tape was stretched by a ratio of 5 in the longitudinal direction at100° C., followed by stretching by a ratio of 9 in the transversedirection at 105° C. after which heat treatment (heat setting) at 135°C. was immediately carried out.

The obtained microporous polyethylene membrane has excellentpermeability to ethanol/water=1/2 solution and is suitable as asubstrate for a composite membrane.

Example 5

The polyethylene solution was prepared in the same way as for Example 1.

A gel-like sheet was prepared by extruding this polyethylene solutioninto sheets using a die at 160° C. then cooling the extrudate in a waterbath at 25° C.

The base tape was dried for 10 minutes at 55° C. and for a further 10minutes at 95° C. to remove decalin from the base tape. Thereafter, thebase tape was stretched by a ratio of 7 in the longitudinal direction at100° C. and then stretched by a ratio of 13 in the transverse directionat 100° C. after which heat treatment (heat setting) at 135° C. wasimmediately carried out. Apart from these differences the microporouspolyethylene membrane was obtained in the same way as in Example 1.

The obtained microporous polyethylene membrane has excellentpermeability to ethanol/water=1/2 solution and is suitable as asubstrate for a composite membrane.

Example 6

A microporous polyethylene membrane was obtained in the same way as inExample 1 except for the following. A polyethylene compositioncomprising a mixture of 6 parts by mass of a high molecular weightpolyethylene (PE1) with a mass-average molecular weight of 4,600,000,and 6 parts by mass of a low molecular weight polyethylene (PE2) with amass-average molecular weight of 560,000 was used, and a polyethylenesolution was prepared so that the concentration of the total amount ofthe polyethylene resin was 12% by mass by mixing with a solvent mixtureof 30 parts by mass of decalin (decahydronaphthalene) and 58 parts bymass of paraffin which was prepared in advance.

A gel-like sheet was prepared by extruding this polyethylene solutioninto sheets using a die at 160° C. then cooling the extrudate in a waterbath at 25° C.

The base tape was dried for 10 minutes at 55° C. and for a further 10minutes at 95° C. to remove decalin from the base tape. Thereafter, thebase tape was stretched by a ratio of 6.5 in the longitudinal directionat 110° C., and then stretched by a ratio of 15 in the transversedirection at 115° C. after which heat treatment (heat setting) at 138°C. was immediately carried out.

The obtained microporous polyethylene membrane has excellentpermeability to ethanol/water=1/2 solution and is suitable as asubstrate for a composite membrane.

Comparative Example 1

A microporous polyethylene membrane was obtained in the same way as inExample 1 except for the following. A polyethylene compositioncomprising a mixture of 3 parts by mass of a high molecular weightpolyethylene (PE1) with a mass-average molecular weight of 4,600,000,and 14 parts by mass of a low molecular weight polyethylene (PE2) with amass-average molecular weight of 560,000 was used, and a polyethylenesolution was prepared so that the concentration of the total amount ofthe polyethylene resin was 17% by mass by mixing with a solvent mixtureof 32 parts by mass of decalin (decahydronaphthalene) and 51 parts bymass of paraffin which was prepared in advance.

A gel-like sheet was prepared by extruding this polyethylene solutioninto sheets using a die at 160° C. then cooling the extrudate in a waterbath at 25° C.

The base tape was dried for 10 minutes at 55° C. and for a further 10minutes at 95° C. to remove decalin from the base tape. Thereafter, thebase tape was stretched by a ratio of 5.5 in the longitudinal directionat 90° C., and then stretched by a ratio of 11 in the transversedirection at 105° C. after which heat treatment (heat setting) at 139°C. was immediately carried out.

The obtained microporous polyethylene membrane has a large porediameter, has poor permeability to ethanol/water=1/2 solution and is notsuitable as a substrate for a composite membrane.

Comparative Example 2

A polyethylene composition comprising a mixture of 3 parts by mass of ahigh molecular weight polyethylene (PE1) with a mass-average molecularweight of 4,600,000, and 27 parts by mass of a low molecular weightpolyethylene (PE2) with a mass-average molecular weight of 560,000 wasused, and a polyethylene solution was prepared such that theconcentration of the total amount of the polyethylene resin was made tobe 30% by mass by mixing with 70 parts by mass of decalin(decahydronaphthalene).

A gel-like sheet was prepared by extruding this polyethylene solutioninto sheets using a die at 160° C. then cooling the extrudate in a waterbath at 20° C.

The gel-like sheet was subjected to preliminary (first) drying in a 70°C. atmosphere for 20 minutes, followed by primary (preliminary)stretching in the longitudinal direction by a ratio of 1.5 at roomtemperature (25° C.). Main drying was performed thereafter for 5 minutesin a 60° C. atmosphere. The remaining solvent in the base tape after themain drying was 20% by mass. After completing the main drying, secondarystretching was performed by stretching the base tape in the longitudinaldirection by a ratio of 5.5 at a temperature of 100° C., followed bystretching in the transverse direction by a ratio of 13 at a temperatureof 125° C. after which heat treatment (heat setting) at 120° C. wasimmediately carried out to obtain a biaxially stretched microporouspolyethylene membrane.

The obtained microporous polyethylene membrane forms a large contactangle using ethanol/water=1/2 solution, is poorly permeable to anaqueous solution of a hydrophilic resin compound and is not suitable asa substrate for a composite membrane.

Comparative Example 3

A polyethylene composition comprising a mixture of 1.7 parts by mass ofa high molecular weight polyethylene with a mass-average molecularweight of 4,600,000, and 19.3 parts by mass of a low molecular weightpolyethylene with a mass-average molecular weight of 560,000 was used,and a polyethylene solution was prepared such that the concentration ofthe total amount of the polyethylene resin was made to be 21% by mass bymixing with 79 parts by mass of decalin (decahydronaphthalene).

A gel-like sheet was prepared by extruding this polyethylene solutioninto sheets using a die at 170° C. then cooling the extrudate in a waterbath at 25° C.

The gel-like sheet was subjected to preliminary (first) drying in a 55°C. atmosphere for 10 minutes, followed by primary (preliminary)stretching in the longitudinal direction by a ratio of 1.6 at 30° C.Main drying was performed thereafter for 5 minutes in a 50° C.atmosphere (amount of remaining solvent was less than 1%). Aftercompleting the main drying, secondary stretching was performed bystretching the base tape in the longitudinal direction by a ratio of 3.5at a temperature of 95° C. and then stretched in the transversedirection by a ratio of 10 at a temperature of 115° C. after which heattreatment (heat setting) at 135° C. was immediately carried out toobtain a biaxially stretched microporous polyethylene membrane.

The obtained microporous polyethylene membrane forms a large contactangle using ethanol/water=1/2 solution, is poorly permeable to anaqueous solution of a hydrophilic resin compound and is not suitable asa substrate for a composite membrane.

Comparative Example 4

A microporous polyethylene membrane was obtained in the same way as inExample 2 other than that the decalin (decahydronaphthalene) was 40parts by mass and the paraffin was 30 parts by mass.

The obtained microporous polyethylene membrane was used as a substratefor a composite membrane that had a thickness of 6 μm, a porosity of43%, an average pore diameter of 40 nm, a contact angle θ1 of 71 degreesand a contact angle θ2 of 65 degrees between a droplet and the surface 1second and 10 minutes after dripping of a mixed solution of ethanol andwater (volume ratio 1/2) thereon, respectively, and a rate of change ofcontact angle of 8%. The obtained microporous polyethylene membrane hada maximum water concentration lower than that of the Examples and wasnot suitable for a substrate for a composite membrane.

Comparative Example 5

A microporous polyethylene membrane was obtained in the same way as inExample 1 except for the following. 8 parts by mass of a high molecularweight polyethylene (PE1) with a mass-average molecular weight of2,000,000 was used, and a polyethylene solution was prepared such thatthe concentration of the total amount of the polyethylene resin was madeto be 8% by mass by mixing with 92 parts by mass of paraffin which wasprepared in advance.

A gel-like sheet was prepared by extruding this polyethylene solutioninto sheets using a die at 200° C. then cooling the extrudate in a waterbath at 50° C.

The base tape was dried for 10 minutes at 55° C. and for a further 10minutes at 95° C. to remove water that had stuck to the base tape.Thereafter, the base tape was stretched by a ratio of 4 in thelongitudinal direction at 120° C., and then stretched by a ratio of 10in the transverse direction at 120° C. after which heat treatment (heatsetting) at 130° C. was immediately carried out. Next the microporouspolyethylene membrane was soaked successively in two separate tankscontaining methylene chloride baths for 30 seconds each while liquidparaffin was extracted therefrom. Note that that the purity of thewashing solvent in the first tank (low)<in the second tank (high),wherein the first tank was on the side where soaking was started and thesecond tank was on the side where soaking was finished. Thereafter, themicroporous polyethylene membrane was obtained by removing the methylenechloride by drying at 45° C., and carrying out annealing treatment whiletransporting over rollers heated to 90° C.

TABLE 1 Com- Com- Com- Com- Com- parative parative parative parativeparative Ex- Ex- Ex- Ex- Ex- Ex- ex- ex- ex- ex- ex- ample ample ampleample ample ample amples amples amples amples amples 1 2 3 4 5 6 1 2 3 45 Com- Decalin (parts by mass) 13 6 2 2 13 30 32 70 79 40 0 positionParafin (parts by mass) 72 64 78 78 72 58 51 0 0 30 92 PE concentration15 30 20 20 15 12 17 30 21 30 8 (% by mass) PE1 (parts by mass) 12 6 1616 12 6 3 3 1.7 6 8 PE1Mw 4.6 4.6 4.6 4.6 4.6 4.6 4.6 4.6 4.6 4.6 2million million million million million million million million millionmillion million PE2 (parts by mass) 3 24 4 4 3 6 14 27 19.3 24 — PE2Mw560,000 560,000 560,000 560,000 560,000 560,000 560,000 560,000 560,000560,000 — Extrusion Die temperature (° C.) 160 160 160 160 160 160 160160 170 160 200 Cooling temperature (° C.) 25 25 25 25 25 25 20 20 25 2550 Drying First drying 55 55 55 55 55 55 55 70 55 55 55 temperature (°C.) First drying time (min.) 10 10 10 10 10 10 10 20 10 10 10 Seconddrying 95 95 95 95 95 95 95 — — 95 95 temperature (° C.) Second dryingtime (min.) 10 10 10 10 10 10 10 — — 10 10 Preliminary Stretchingtemperature — — — — — — — 25 30 — — stretching (° C.) Stretching ratio(ratio) — — — — — — — 1.5 1.6 — — Main Temperature (° C.) — — — — — — —60 50 — — drying Time (min.) — — — — — — — 5 5 — — StretchingLongitudinal stretching 100 100 100 100 100 110 90 100 95 100 120temperature (° C.) Longitudinal stretching 5.5 5.5 3.9 5 7 6.5 5.5 5.53.5 5.5 4 ratio (ratio) Transverse stretching 110 110 100 105 100 115105 125 115 110 120 temperature (° C.) Transverse stretching 13 13 13 913 15 11 13 10 13 10 ratio (ratio) Heat setting temperature 135 125 135135 135 138 139 120 135 125 130 (° C.) Area ratio (ratio) 72 72 51 45 9198 61 72 35 72 40 Extracting Extracting time (sec.) 60 60 60 60 60 60 60— — 60 60 Drying temperature (° C.) 45 45 45 45 45 45 45 — — 45 45Annealing temperature 120 120 120 120 120 120 120 — — 120 90 (° C.)

TABLE 2 Tensile Tensile Average Gurley breaking breaking Contact angle(degrees) Thick- Poro- pore value strength strength 1 10 Contact nesssity size [sec./ MD TD second min. angle rate [um] [%] [nm] 100 cc][Mpa] [Mpa] after after change % Example 1 6 66 31 30 150 180 84 49 41Example 2 11 50 20 85 130 220 78 65 17 Example 3 5 55 25 70 130 180 7359 20 Example 4 12 55 30 90 140 160 78 57 26 Example 5 6 63 50 36 190180 63 52 40 Example 6 8 78 35 30 55 110 80 45 44 Com- 11 58 60 60 25 3083 60 27 parative examples 1 Com- 27 88 1000 3.5 10 14 120 114 5parative examples 2 Com- 12 85 500 4 7 10 115 108 6 parative examples 3Com- 6 43 40 90 120 150 71 65 8 parative examples 4 Com- 12 80 60 45 4570 77 70 9 parative examples 5 Maximum water concentration MaximumSurface ET free con- Actual energy Permeability of ethanol-watersolution centration EtOH of liquid 30% 31% 32% 33% 34% 35% 36% 37% 38%39% 40% vol % wt % vol % (mJ/m²) Example 1 x ∘ ∘ ∘ ∘ ∘ ∘ ∘ ∘ ∘ ∘ 29.5%23.1% 70.6% 36.5 Example 2 x x x ∘ ∘ ∘ ∘ ∘ ∘ ∘ ∘ 31.4% 24.6% 68.7% 35.8Example 3 x x x ∘ ∘ ∘ ∘ ∘ ∘ ∘ ∘ 31.4% 24.6% 68.7% 35.8 Example 4 x x x x∘ ∘ ∘ ∘ ∘ ∘ ∘ 32.3% 25.4% 67.7% 35.4 Example 5 x x ∘ ∘ ∘ ∘ ∘ ∘ ∘ ∘ ∘30.4% 23.9% 69.6% 36.2 Example 6 x ∘ ∘ ∘ ∘ ∘ ∘ ∘ ∘ ∘ ∘ 29.5% 23.1% 70.6%36.4 Com- x x x x x x x x x x ∘ 38.0% 29.8% 62.0% 33.0 parative examples1 Com- x x x x x x x x x x x 38.0% 29.8% 62.0% 33.0 parative examples 2Com- x x x x x x x x x x x 38.0% 29.8% 62.0% 33.0 parative examples 3Com- x x x x x x x x x ∘ ∘ 37.1% 29.1% 63.0% 33.5 parative examples 4Com- x x x x x x x x ∘ ∘ ∘ 36.1% 28.3% 63.9% 34.0 parative examples 5

(Verification by Application in Solid Polymer Fuel Cell)

A substrate for a composite membrane comprising the microporouspolyethylene membrane of Example 1 was used to make an electrolytemembrane of a solid polymer fuel cell in which a perfluorosulfonic acidresin was carried in the pores thereof and the effect was verified.

(1) Examination of Solvent Ratio

A substrate for a composite membrane (white opaque membrane) comprisingthe microporous polyethylene membrane of Example 1 was placed on a glassplate, a mixed solution of water/ethanol was gently dripped from above,and the change in color of the substrate was confirmed. The followingthree types of mixed solution were used. A: water/ethanol=4/1 (massratio), B: water/ethanol=3/1 (mass ratio), and C: water/ethanol=2/1(mass ratio). As a result, as only the mixed solution C(water/ethanol=2/1 (mass ratio)) changed the color of the substrate totransparent, it could be confirmed that the pores of the substrate wereloaded with the mixed solution. Note that the color of the substratewhen mixed solutions A and B were used remained an opaque white and sothe mixed solution was not loaded into the pores of the substrate. Inthe following examination, an electrolyte membrane was prepared usingthe mixed solution C (water/ethanol=2/1 (mass ratio)).

(2) Preparation of Electrolyte Membrane and MEA of Solid Polymer FuelCell

A substrate for a composite membrane comprising the microporouspolyethylene membrane of Example 1 was used as a substrate and aperfluorosulfonic acid resin (equivalent mass EW=560) was used as thehydrophilic resin compound. A polymer solution was prepared bydissolving perfluorosulfonic acid resin into the mixed solution ofwater/ethanol=2/1 (mass ratio) such that the polymer concentration was3.3% by mass. After impregnating the substrate with the polymersolution, drying was performed at room temperature to remove thesolvent. Furthermore, after annealing treatment for 13.5 hours at 100°C., washing and drying were performed and an electrolyte membrane(membrane thickness of about 7 μm) made of the composite membrane wasobtained.

(Evaluation of Fuel Cell Operation)

A catalyst paste was prepared by putting 10.84 g of a dispersioncomposition of the perfluorosulfonic acid resin (EW=560), 2.0 g of TKKPt/C (Tanaka Holdings Co., Ltd., TEC10E50E, platinum loading amount of45.9%), 8.67 g of RO water, 8.67 g of 1-propanol and 8.67 g of2-propanol into a zirconia container together with 200 g of zirconiaballs (φ 5), and mixing using a planetary ball mill (manufactured byFritz of Germany) at a rotation speed of 200 rpm for 1 hour.

The electrode catalyst layer was prepared by coating the catalyst pasteprepared as described above on a polytetrafluoroethylene (PTFE) sheetwith an applicator PI-1210 (Tester Sangyo) and drying in an airatmosphere. The amount of platinum loaded was adjusted to around 0.3mg/cm².

The MEA was prepared by sandwiching an electrolyte membrane between twoof the aforementioned electrode catalyst layers cut out to 5 cm²,hot-pressing at 135° C. and a pressure of 2.0 kN for 1 minute, thenpeeling off the PTFE sheet.

Both sides of the MEA were sandwiched between gas diffusion layers(SIGRACET GDL 24 BC, manufactured by SGL GROUP), and assembled into asingle cell (catalytic layer area: 5 cm²) made by ElectroChem togetherwith a gasket. The cell temperature was set to 80° C., a water bubblingmethod was used to control the relative humidity of gas flowing to bothelectrodes, and two types of electrochemical characteristics weremeasured. One involved a current interrupt method, in which hydrogen gaswas supplied to the anode side and oxygen gas was supplied to thecathode side at flow rates of 100 mL/min and 500 mL/min, respectively,and the relative humidity at both electrodes were simultaneously changedto 60% RH, 30% RH, 20% RH and 10% RH. Using an electrochemicalmeasurement system HZ-3000 (Hokuto Denko Co., Ltd.) with an initialstate of 1 A/cm², a current was passed through the cell for 1 minute andthe ohmic resistance was calculated by measuring the voltage change whenthe current was momentarily interrupted. The second involved I-Vcharacteristic test, in which hydrogen gas was supplied as fuel to theanode side and oxygen gas or air was supplied as an oxidizing agent tothe cathode side at flow rates of 100 mL/min and 500 mL/min,respectively, and the relative humidity at both electrodes weresimultaneously changed to 30% RH, 20% RH, 10% RH. The cell voltage wasmeasured when the current was driven from 0 to 10 A with a batterycharging and discharging device HJ 1010 SMSA (Hokuto Denko Corporation).

FIG. 1 shows, with respect to the MEA acquired as described above, theresults of calculating the degree of proton conductivity of the MEA bycalculating the ohmic resistance with a current interrupter. As areference example, Nafion NR 211 (membrane thickness of 25 μm) was used,which is an electrolyte membrane manufactured by Du Pont. As shown inFIG. 1, with respect to proton conductivity, in addition to the effectof thinning the membrane thickness to about one quarter, as a low EWperfluorosulfonic acid polymer having a high proton conductivity isloaded therein, the MEA prepared using the substrate of the presentinvention showed higher performance than when NR 211 was used.

FIG. 2 shows the relationship between the cell voltage and the currentdensity in the MEA obtained as described above at a humidity of 20% orless (oxidizing agent: O₂ or air). As a reference example, Nafion NR 211(membrane thickness of 25 μm) was used, which is an electrolyte membranemanufactured by Du Pont. As shown in FIG. 2, in a low humidityenvironment of 20% humidity, almost no electricity generation waspossible with the conventional NR 211, whereas the MEA produced usingthe substrate of the present invention could generate electricity at upto 2 A/cm². Thus, it is understood that a novel electrolyte membrane wasobtained. Based on the fact that a low EW perfluorosulfonic acid polymer(EW 560) having a high proton conductivity was used for the ionomer inthe catalyst layer, that an electrolyte membrane was prepared by loadingthe microporous polyolefin membrane with the electrolyte of EW 560, andthat the electrolyte membrane was thinned, it is considered that watergenerated at the cathode could sufficiently permeate to the anode sideof the electrolyte membrane so that the humidity inside the electrolytemembrane could be maintained.

INDUSTRIAL APPLICABILITY

The substrate for a composite membrane according to the presentinvention has industrial applicability in that as the pores of thesubstrate can be suitably loaded with a perfluorosulphonic acid typeresin to obtain a very thin composite membrane, a novel electrolytemembrane capable of generating electricity even under low humidityenvironments which has more excellent proton conductivity than beforecan be provided.

What is claimed is:
 1. A composite membrane comprising a microporouspolyolefin membrane and a hydrophilic resin compound carried withinpores of the microporous polyolefin membrane wherein: the average poresize of the microporous polyolefin membrane is 1 nm to 50 nm; theporosity of the microporous polyolefin membrane is 50% to 78%; theGurley value as measured according to HS P8117 is 90 s/100 cc or less;and the membrane thickness of the microporous polyolefin membrane is 1μm to 10 μm.
 2. The composite membrane according to claim 1, whereinwhen a mixed solution of ethanol and water (volume ratio 1/2) is drippedonto a surface of the microporous polyolefin membrane which has notundergone hydrophilization treatment, a contact angle θ1 between thedroplet and the surface is 0 to 90 degrees 1 second after the dripping,and a contact angle θ2 between the droplet and the surface is 0 to 70degrees 10 minutes after the dripping.
 3. The composite membraneaccording to claim 2, wherein the rate of change of the contact angle((θ1−θ2)/θ1×100) is 10 to 50%.
 4. The composite membrane according toclaim 1, wherein the porosity of the microporous polyolefin membrane is60% to 78%.
 5. The composite membrane according to claim 1, wherein thepolyolefin is a polyethylene composition comprising a high molecularweight polyethylene with a mass average molecular weight of 900,000 ormore and a low molecular weight polyethylene with a mass averagemolecular weight of 200,000 to 800,000 mixed at a mass ratio of 20:80 to80:20.
 6. The composite membrane according to claim 1, wherein themicroporous polyolefin membrane can be impregnated with a liquid whichis a solvent for the hydrophilic resin compound and has a surface freeenergy of 35 to 36.5 mJ/m².
 7. The composite membrane according to claim6, wherein the microporous polyolefin membrane can be impregnated with amixture of ethanol and water in which the water concentration is greaterthan 65.8% by volume to 70.6% by volume or less.
 8. The compositemembrane according to claim 1, wherein the tensile breaking strength (MDor TD) per unit cross sectional area of the polyolefin solid content ofthe microporous polyolefin membrane is 50 MPa or more.
 9. The compositemembrane according to claim 1, wherein the membrane thickness is 1 μm to6 μm.